drm/i915: create the DDI encoder

Now intel_ddi_init is just like intel_hdmi_init and intel_dp_init: it
inits the encoder and then calls the proper init_connector functions.
Notice that for non-eDP ports we call both HDMI and DP connector init,
so we have 2 connectors attached to each DDI encoder.

After this change, intel_hdmi_init and intel_dp_init are only called
by Ivy Bridge and earlier, while hardware containing DDI outputs
should call intel_ddi_init.

Also added/removed quite a few "static" keywords due to the fact that
some function pointers were moved from intel_dp.c and intel_hdmi.c to
intel_ddi.c.

DP finally works on Haswell now! \o/

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>